This invention relates generally to machinery used for welding together plastic parts, and more particularly, to an improved apparatus for holding a plastic part in position during the welding process.
A variety of machines are known which are capable of welding together two or more plastic parts. These machines have been found to be well suited for use in the manufacture of plastic battery cases, or jars. An example of a plastic welding machine which has been used for this purpose, and in particular to weld plastic battery covers to plastic battery cases, is manufactured by Bielomatik Plastic Welding Machines. In operating such an apparatus, a battery case is first delivered to the welding machine with a battery cover loosely placed in position on top of the case. The case is then raised into position, bringing the cover into contact with a holding device which is adapted to engage the cover and hold it in place over the battery case. The battery case is then lowered to its original position, thereby retaining the cover in a position spaced apart from the case. A heated platen is then inserted between the separated cover and case. The corresponding edges of the cover and the case which are to be welded together are then planned against the heated platen in order to soften the plastic. The platen is then removed from between the cover and case, which are then pressed together in order to effect the case to cover weld.
The present arrangement used to hold the cover in place employs a vacuum applied to the cover through an arrangement of passages and suction cups built into the cover holder. A vacuum pump is provided with the welding machine to supply the required vacuum. Through the suction cups, a partial vacuum seal is produced between the cover holder and the cover, allowing the cover to be lifted. It has been found that this arrangement is not entirely satisfactory.
For example, problems often arise relating to machine cycle interruptions caused by improper alignment and sealing between the holding device and the covers. At times, the design of the cover itself contributes to an improper seal, in that the cover may incorporate certain design features which are incompatible with the existing cover holding system.
Although mechanical, hydraulic type cover holders are available, such devices also exhibit several drawbacks. For example, such holding devices present additional costs. Further, hydraulic operation of the device necessitates extensive modifications to the existing equipment, since means for hydraulic operation are not provided with the existing machinery. In addition, there is created a potential for leakage of hydraulic fluid, which could adversely affect the product, as well as the conveyor system itself.
Accordingly, there is presented a real need for a holding device which is easily adapted for use in conjunction with presently existing plastic welding machinery, but which overcomes the problems presented by the cover holding devices which are currently in use.